Structure and manufacturing method of the structure, and gallium nitride-based semiconductor light-emitting device using the structure and manufacturing method of the device

ABSTRACT

In a structure including a gallium nitride-based semiconductor having an m-plane as a principal plane, and a metal layer provided on the principal plane, the principal plane has an n-type conductivity. An interface between the gallium nitride-based semiconductor and the metal layer contains oxygen. The metal layer includes a crystal grain extending form a lower surface to an upper surface of the metal layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of International Application No.PCT/JP2013/005281 filed on Sep. 5, 2013, which claims priority toJapanese Patent Application No. 2012-219533 filed on Oct. 1, 2012. Theentire disclosures of these applications are incorporated by referenceherein.

BACKGROUND

The present disclosure relates to structures each of which includes agallium nitride-based semiconductor layer having a surface that is anonpolar plane or a semi-polar plane, and includes a metal layerprovided on the surface of the gallium nitride-based semiconductorlayer.

Nitride semiconductors containing nitrogen (N) as a group V element havebeen expected as a material of a short wavelength light-emitting elementbecause of their band gap size. In particular, gallium nitride-basedcompound semiconductors (GaN-based semiconductors) containing Ga as agroup III element have been intensively studied, and blue light-emittingdiode (LED) elements, green LED elements, and semiconductor laserelements formed of GaN-based semiconductors have also beencommercialized.

GaN-based semiconductors have a wurtzite crystal structure. FIG. 1schematically illustrates a unit lattice of GaN. In Al_(x)Ga_(y)In_(z)Nsemiconductor crystal (where 0≦x<1, 0<y≦1, 0≦z<1, and x+y+z=1), some ofGa atoms illustrated in FIG. 1 may be substituted with at least one ofAl or In.

FIG. 2 shows fundamental vectors a₁, a₂, a₃ and c of the wurtzitecrystal structure. The fundamental vector c extends in a [0001]direction, and this direction is referred to as a “c-axis.” A planeperpendicular to the c-axis is referred to as a “c-plane” or a “(0001)plane.” A plane terminated with group III elements, such as Ga, isreferred to as a “+c-plane” or a “(0001) plane,” and a plane terminatedwith group V elements, such as nitrogen, is referred to as a “−c-plane”or a “(000-1) plane,” and these planes are distinguished from eachother.

In the case where a semiconductor element is fabricated using aGaN-based semiconductor, in general, a c-plane substrate, that is, asubstrate having a (0001) plane as a growth surface is used as asubstrate on which a GaN-based semiconductor crystal is grown. However,in the c-plane, Ga atoms and nitrogen atoms are not present on the sameatomic plane, and therefore, electrical polarization occurs. For thisreason, the “c-plane” is also referred to as a “polar plane.” As aresult of the electrical polarization, a piezoelectric field isgenerated along the c-axis in an InGaN quantum well layer included in anactive layer of the gallium nitride-based semiconductor light-emittingdevice. Due to the piezoelectric field generated in the active layer,electrons and holes distributed in the active layer are displaced, andthe internal quantum efficiency of the active layer is decreased due toa quantum-confined Stark effect of carriers. This increases a thresholdcurrent in the case of a semiconductor laser element. This alsoincreases power consumption and reduces luminous efficiency in the caseof an LED. This further increases implanted carrier concentration,piezoelectric field screening, and a change in light emissionwavelength.

To solve these problems, using a substrate (an m-plane GaN-basedsubstrate) having a nonpolar plane as its growth surface, e.g., a(10-10) plane called an m-plane that is perpendicular to the [10-10]direction, has been considered. The sign “-” given to the left side ofan index of Miller indices in parentheses indicates a “bar (inversion)”of that index, and corresponds to the “bar” in the drawing. Asillustrated in FIG. 2, m-plane is in parallel with c-axis and isorthogonal to c-plane. In the m-plane, Ga atoms and nitrogen atoms arepresent on the same atomic plane, and therefore, spontaneous electricalpolarization does not occur in a direction perpendicular to the m-plane.This means that a piezoelectric field is not generated in the activelayer, and the above problems are solved, if a stacking semiconductorstructure is formed in the direction perpendicular to the m-plane. Them-plane is a collective term for (10-10) plane, (−1010) plane, (1-100)plane, (−1100) plane, (01-10) plane, and (0-110) plane.

As illustrated in FIG. 3C, a-plane is in parallel with the c-axis (afundamental vector c) and is orthogonal to the c-plane illustrated inFIG. 3A. The a-plane is a collective term for (11-20) plane, (−1-120)plane, (1-210) plane, (−12-10) plane, (−2110) plane, and (2-1-10) plane.

FIG. 3D illustrates r-plane, which is a collective term for (10-12)plane, (−1012) plane, (1-102) plane, (−1102) plane, (01-12) plane, and(0-112) plane.

Further, −r-plane is a collective term for (10-1-2) plane, (−101-2)plane, (1-10-2) plane, (−110-2) plane, (01-1-2) plane, and (0-11-2)plane.

Japanese Unexamined Patent Publication No. 2005-197687 discloses atechnique that uses a structure of an antioxidation electrode/anaggregation-prevention electrode/a reflecting electrode/a contactelectrode/a p-type GaN, to prevent aggregation of the reflectingelectrode formed of silver (Ag), rhodium (Rh), aluminum (Al) or tin(Sn).

Jun Ho Son, Yang Hee Song, Hak Ki Yu, and Jong-Lam Lee, “Applied PhysicsLetters” Vol. 95, P. 062108, Aug. 14, 2009 discloses a technique inwhich in order to increase a reflection coefficient of an Ag electrodeand reduce contact resistance, a nickel (Ni) layer is formed on aninterface between a GaN layer and an Ag layer, thereby promotingcrystallization of Ag.

Japanese Unexamined Patent Publication No. 2010-56423 discloses anelectrode for a semiconductor light-emitting device, and the electrodeincludes an Ag alloy layer added with palladium (Pd) and copper (Cu) orgermanium (Ge), using Ag as a principal component, to achieve both of ahigh reflection coefficient and a low contact resistance of theelectrode.

Japanese Unexamined Patent Publication No. 2010-062274 discloses asemiconductor light-emitting diode which includes: a stacking structureincluding an n-type semiconductor layer, a p-type semiconductor layer,and a light-emitting layer provided between the n-type semiconductorlayer and the p-type semiconductor layer; a first electrode connected tothe n-type semiconductor layer and containing at least one of silver ora silver alloy; and a second electrode connected to the p-typesemiconductor layer.

Japanese Unexamined Patent Publication No. 2012-080142 discloses amethod for manufacturing a semiconductor light-emitting diode whichincludes: a stacking structure including a first semiconductor layer ofa first conductivity type, a second semiconductor layer of a secondconductivity type, and a light-emitting layer provided between the firstsemiconductor layer and the second semiconductor layer; and an electrodeprovided on the second semiconductor layer disposed opposite to thelight-emitting layer. In this publication, a first metal layercontaining silver or a silver alloy is formed on a surface of the secondsemiconductor layer disposed opposite to the light-emitting layer, and asecond metal layer containing at least one element of platinum,palladium and rhodium is formed on the first metal layer. The secondsemiconductor layer, the first metal layer, and the second metal layerare sintered in an atmosphere containing oxygen. The sinteringtemperature is such a temperature that makes an average particlediameter of silver contained in the sintered first metal layer is notmore than three times an average particle diameter of the silver beforesintering.

Japanese Unexamined Patent Publication No. H10-200161 disclosesperforming oxygen plasma ashing on a surface of an n-type GaN contactlayer, and thereafter forming an electrode in which a Ti layer, an Allayer, a Pt layer, and an Au layer are sequentially formed.

SUMMARY

According to the above conventional techniques, however, furtherimprovement of the external quantum efficiency has been demanded.

An objective of a non-limiting example embodiment of the presentapplication is to improve a reflection coefficient and a contactresistance of a metal layer, and thereby increase the external quantumefficiency.

To achieve the above objective, an aspect of the present disclosure isdirected to a structure including a gallium nitride-based semiconductorlayer having an m-plane as a principal plane and a silver layer providedon the principal plane, wherein the principal plane has an n-typeconductivity, and an interface between the gallium nitride-basedsemiconductor layer and the silver layer contains oxygen, and the silverlayer includes a crystal grain extending from a lower surface to anupper surface of the silver layer.

An embodiment of the present disclosure improves a reflectioncoefficient and a contact resistance of a metal layer, and increases theexternal quantum efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an oblique view schematically showing a unit lattice of GaN.

FIG. 2 is an oblique view showing fundamental vectors a₁, a₂, a₃, and cof a wurtzite crystal structure.

FIG. 3A to FIG. 3D schematically shows typical crystalline planeorientation of a hexagonal wurtzite structure.

FIG. 4 is a cross-sectional view of a structure including a galliumnitride-based semiconductor layer and a metal layer of the firstembodiment.

FIG. 5A is a cross-sectional view of a gallium nitride-basedsemiconductor light-emitting device of the second embodiment. FIG. 5B isa cross-sectional view of a gallium nitride-based semiconductorlight-emitting device of the first variation of the second embodiment.FIG. 5C is a cross-sectional view of a gallium nitride-basedsemiconductor light-emitting device of the second variation of thesecond embodiment.

FIG. 6A is a cross-sectional view of a gallium nitride-basedsemiconductor light-emitting device of the third embodiment. FIG. 6B isa cross-sectional view of a gallium nitride-based semiconductorlight-emitting device of the first variation of the third embodiment.FIG. 6C is a cross-sectional view of a gallium nitride-basedsemiconductor light-emitting device of the second variation of the thirdembodiment.

FIG. 7A shows a structure of a comparative example of the first example,and is an optical microscope image of a surface of an Al layer beforeheat treatment. FIG. 7B shows a structure of a comparative example ofthe first example, and is an optical microscope image of the surface ofthe Al layer after heat treatment.

FIG. 8A is a structure of a comparative example of the first example,and is an optical microscope image of an interface between an m-planen-type GaN layer and an Al layer before heat treatment. FIG. 8B shows astructure of a comparative example of the first example, and is anoptical microscope image of an interface between m-plane n-type GaN andan Al layer after heat treatment.

FIG. 9A shows a structure including a gallium nitride-basedsemiconductor layer and a metal layer of the first example, and is anoptical microscope image of a surface of the Al layer before heattreatment. FIG. 9B shows a structure of the first example, and is anoptical microscope image of the surface of the Al layer after heattreatment.

FIG. 10A shows a structure including a gallium nitride-basedsemiconductor layer and a metal layer of the first example, and is anoptical microscope image of an interface between an m-plane n-type GaNlayer and an Al layer before heat treatment. FIG. 10B shows a structureof the first example, and is an optical microscope image of an interfacebetween an m-plane n-type GaN layer and an Al layer after heattreatment.

FIG. 11 is a graph showing the current-voltage characteristics of astructure including a gallium nitride-based semiconductor layer and ametal layer of a comparative example of the first example.

FIG. 12 is a graph showing the current-voltage characteristics of astructure including a gallium nitride-based semiconductor layer and ametal layer of the first example.

FIG. 13 is a graph showing the current-voltage characteristics of astructure including a gallium nitride-based semiconductor layer and ametal layer of a comparative example of the second example.

FIG. 14 is a graph showing the current-voltage characteristics of astructure including a gallium nitride-based semiconductor layer and ametal layer of the second example.

FIG. 15 is a graph relating to a comparative example of the secondexample, and shows measurement results of an interface reflectioncoefficient between an m-plane n-type GaN layer and an Ag layer afterheat treatment.

FIG. 16 is a graph relating to a structure of the second exampleincluding a gallium nitride-based semiconductor layer and a metal layer,and shows measurement results of the interface reflection coefficientbetween an m-plane n-type GaN layer and an Ag layer after heattreatment.

FIG. 17A shows a structure of a comparative example of the secondexample, and is an optical microscope image of an interface between anm-plane n-type GaN layer and an Al layer before heat treatment. FIG. 17Bshows a structure of a comparative example of the second example, and isan optical microscope image of an interface between an m-plane n-typeGaN layer and an Al layer after heat treatment.

FIG. 18A shows a structure of the second example including a galliumnitride-based semiconductor layer and a metal layer, and is an opticalmicroscope image of an interface between an m-plane n-type GaN layer andan Al layer before heating. FIG. 18B shows a structure of the secondexample, and is an optical microscope image of an interface between anm-plane n-type GaN layer and an Al layer after heat treatment.

FIG. 19 shows a structure of a comparative example of the secondexample, and is a cross-sectional transmission electron microscope (TEM)image of an interface between an m-plane n-type GaN layer and an Aglayer after heat treatment.

FIG. 20 shows a structure of the second example including a galliumnitride-based semiconductor layer and a metal layer, and is across-sectional TEM image of an interface between an m-plane n-type GaNlayer and an Ag layer after heat treatment.

FIG. 21 shows a structure of a comparative example of the secondexample, and is a cross-sectional transmission electron microscope (TEM)image of an interface between an m-plane n-type GaN layer and an Aglayer after heat treatment.

FIG. 22 shows a structure of the second example including a galliumnitride-based semiconductor layer and a metal layer, and is across-sectional TEM image of an interface between an m-plane n-type GaNlayer and an Ag layer after heat treatment.

FIG. 23 is a cross-sectional view schematically showing thecross-sectional TEM image of FIG. 19.

FIG. 24 is a cross-sectional view schematically showing thecross-sectional TEM image of FIG. 20.

FIG. 25 is a flow chart showing an example method for manufacturing astructure including a gallium nitride-based semiconductor layer and ametal layer according to another embodiment.

FIG. 26 is a flow chart showing an example method for manufacturing agallium nitride-based semiconductor light-emitting device according toanother embodiment.

FIG. 27 is a flow chart showing an example method for manufacturing agallium nitride-based semiconductor light-emitting device according tostill another embodiment.

DETAILED DESCRIPTION

In the invention disclosed in Japanese Unexamined Patent Publication No.2005-197687, Ag, Rh, Al or Sn is used for the reflecting electrode, andthe contact electrode needs to be selected from the group consisting ofan La-based alloy, an Ni-based alloy, a Zn-based alloy, a Cu-basedalloy, a thermoelectric oxide, a doped In oxide, ITO, and ZnO. Theprocess may thus be complicated.

In the invention disclosed in Jun Ho Son, Yang Hee Song, Hak Ki Yu, andJong-Lam Lee, “Applied Physics Letters” Vol. 95, P. 062108, Aug. 14,2009, an Ni layer, i.e., a dissimilar metal, needs to be insertedbetween a GaN layer and an Ag layer.

In the invention disclosed in Japanese Unexamined Patent Publication No.2010-56423, dissimilar elements, such as Pd and Cu or Ge, needs to beadded to Ag. The process may thus be complicated in this case as well.

In an embodiment of the present disclosure, it is possible to provide agallium nitride-based semiconductor light-emitting device of which theexternal quantum efficiency is greater than the external quantumefficiencies of the conventional techniques described above.

In order to extract light from a semiconductor light-emitting devicewith high efficiency, it is necessary to reduce as much light that isabsorbed inside and outside a light-emitting element as possible.Depositing Al or Ag, which is a metal with a high reflectioncoefficient, or an alloy containing Al and Ag, on a semiconductor layermay be a way of reducing absorption of light. Such a metal layer can beused as an electrode which allows a current to flow in thelight-emitting element. However, in some of combinations ofsemiconductor and metal, an ohmic contact may fail to be formed and thepower conversion efficiency may be reduced. Further, such a metal layerhas a low resistance to heat and other disturbances, and an interfacereflection coefficient between the semiconductor layer and the metallayer is low. Thus, light-extraction efficiency may be reduced in somecases.

In the embodiment of the present disclosure, the affinity between asemiconductor layer and a metal layer becomes stronger, and thesemiconductor layer and the metal layer form a definite, or planarinterface, by modifying the surface of the semiconductor. As a result, areflection coefficient is improved and the long-term reliability is alsoimproved. Moreover, it is possible to form an ohmic contact betweensemiconductor and metal between which an ohmic contact could not beformed by conventional techniques.

For example, a structure according to an embodiment is a structureincluding: a gallium nitride-based semiconductor layer having an m-planeas a principal plane; and a silver layer provided on the principalplane. The principal plane has an n-type conductivity. An interfacebetween the gallium nitride-based semiconductor layer and the silverlayer contains oxygen. The silver layer includes a crystal grainextending from a lower surface to an upper surface of the silver layer.

In one embodiment, surface plasmon resonance absorption is preferablynot observed in light reflected by the interface between the galliumnitride-based semiconductor layer and the silver layer, within awavelength range of from 450 nm to 500 nm.

In one embodiment, the silver layer may have a thickness of not lessthan 200 nm.

In one embodiment, the silver layer may have a thickness of not morethan 1200 nm.

In one embodiment, the crystal grain may have a maximum length of notless than 200 nm.

In one embodiment, the crystal grain may have a maximum length of notmore than 1200 nm.

In one embodiment, an oxygen concentration at the interface between thegallium nitride-based semiconductor layer and the silver layer may benot less than 30 times and not more than 200 times an oxygenconcentration in the silver layer.

In one embodiment, an oxygen concentration at the interface between thegallium nitride-based semiconductor layer and the silver layer may benot less than 3×10²⁰ cm⁻³ and not more than 2×10²¹ cm⁻³, and an oxygenconcentration in the silver layer may be not more than 1×10¹⁹ cm⁻³.

In one embodiment, an arithmetic average roughness Ra of the interfaceof the silver layer with the gallium nitride-based semiconductor layerin an in-plane direction may be not less than 0.27 nm and not more than2.65 nm in the case of a reference length of 3.5 μm.

A gallium nitride-based semiconductor light-emitting device of anotherembodiment is a gallium nitride-based semiconductor light-emittingdevice including: the structure of one embodiment; a p-type galliumnitride-based semiconductor layer, and a light-emitting layer sandwichedbetween the gallium nitride-based semiconductor layer and the p-typegallium nitride-based semiconductor layer, wherein the galliumnitride-based semiconductor layer is an n-type gallium nitride-basedsemiconductor layer, and the silver layer is an electrode for the n-typegallium nitride-based semiconductor layer.

A structure manufacturing method of another embodiment includes: a stepof exposing a surface of the gallium nitride-based semiconductor layerto an atmosphere containing active oxygen, and a step of forming thesilver layer on the surface of the gallium nitride-based semiconductorlayer.

In another embodiment, the method may further include a step ofperforming heat treatment on the structure after forming the silverlayer.

In another embodiment, the step of exposing to the atmosphere containingthe active oxygen may include a step of irradiating the surface of thegallium nitride-based semiconductor layer with ultraviolet light under acondition where the surface is exposed to a gas or a liquid containingan oxygen atom or an oxygen molecule.

In another embodiment, the step of exposing to the atmosphere containingthe active oxygen may include a step of exposing the surface of thegallium nitride-based semiconductor layer to an atmosphere containingoxygen plasma.

Still another embodiment is directed to a method for manufacturing agallium nitride-based semiconductor light-emitting device using themethod for manufacturing the structure. The gallium nitride-basedsemiconductor layer is an n-type gallium nitride-based semiconductorlayer. The method for manufacturing a gallium nitride-basedsemiconductor light-emitting device includes a step of manufacturing, onthe n-type gallium nitride-based semiconductor layer, a stackingsemiconductor structure including a light-emitting layer and a p-typegallium nitride-based semiconductor layer.

The first embodiment will be described below with reference to thedrawings.

First Embodiment

A structure which includes a gallium nitride-based semiconductor layerand a metal layer according to the first embodiment will be describedwith reference to FIG. 4.

At first, as shown in FIG. 4, an n-type gallium nitride-basedsemiconductor 41 is prepared. The n-type gallium nitride-basedsemiconductor 41 may be simply referred to as a “semiconductor 41.” Thesemiconductor 41 is a semiconductor layer having a crystal plane, otherthan a c-plane, as a growth surface, or is part of a stackingsemiconductor structure. In the present embodiment, an uppermost surfaceof the semiconductor 41 which is a crystal plane other than the c-planeis modified (surface modification) through an oxidation reaction,thereby controlling the wettability of the uppermost surface.Specifically, for example, the uppermost surface of the semiconductor 41is exposed to an oxygen plasma atmosphere to increase the hydrophilicityof the uppermost surface. According to the evaluation by the inventorsof the present application, a gallium nitride-based semiconductor ishydrophilic, in general, but effects of the present embodiment can beincreased by controlling the wettability of the gallium nitride-basedsemiconductor to be closer to “super-hydrophilicity.”

The crystal plane other than the c-plane is, for example, a crystalplane tilted at the angle of not less than 18° and not more than 90°with respect to a c-axis of the GaN-based semiconductor. The wettabilityof the gallium nitride-based semiconductor can be much closer to“super-hydrophilicity” by applying the present embodiment to a crystalplane tilted at the angle of not less than 18° and not more than 90°with respect to the c-axis of the GaN-based semiconductor. This may becaused by an atomic structure of the surface of the GaN-basedsemiconductor. In sp³ hybrid orbitals, atomic bonds form an angle of108°. Thus, not less than two atomic bonds exist in the crystal plane ofthe GaN-based semiconductor which is tilted at the angle of not lessthan 18°, obtained by subtracting 90° from the 108°, with respect to thec-axis. This means that the atomic structure of the crystal plane isdifferent from the atomic structure of the c-plane. Both of a surface ofan m-plane GaN-based semiconductor and a surface of an a-plane GaN-basedsemiconductor are tilted with respect to the c-axis of the GaN-basedsemiconductor by 90°, which is in the above range. Both of a surface ofa −r-plane GaN-based semiconductor and a surface of a +r-plane GaN-basedsemiconductor are tilted with respect to the c-axis of the GaN-basedsemiconductor by about 43°, which is in the above range.

The “m-plane” of the present embodiment includes not only a plane thatis completely parallel to an non-tilted m-plane, but also a plane thatis tilted within ±5° in either direction with respect to the non-tiltedm-plane. Effects of spontaneous electrical polarization are very smallin the case where the plane is slightly tilted with respect to them-plane. In crystal growth techniques, there are cases in which asemiconductor layer can be epitaxially grown more readily on a substratehaving a surface slightly tilted with respect to crystal orientationthan on a substrate having a surface strictly aligned to the crystalorientation. Thus, in some cases, it is effective to tilt a crystalplane that is a surface on which a semiconductor layer is epitaxiallygrown, in order to improve the quality of the semiconductor layer andincrease a crystal growth rate, while sufficiently reducing effects ofthe spontaneous electrical polarization.

Further, the plane slightly tilted as a whole with respect to thenon-tilted m-plane has a lot of step-like, exposed m-plane areas havingproperties similar to those of the m-plane. The “m-plane” of the presentdisclosure includes a plane which has a plurality of step-like m-planeareas.

Further, a plane that is tilted within ±5° in either direction withrespect to a non-tilted a-plane, +r-plane, −r-plane, S-plane ((10-11)plane), n-plane ((11-23) plane), R-plane ((10-14) plane), (11-22) plane,(20-21) plane, (10-13) plane, (20-2-1) plane, or (10-1-3) plane, etc.,has properties similar to those of its corresponding non-tilted plane.Thus, the “a-plane,” “+r-plane,” “−r-plane,” “S-plane,” “n-plane,”“R-plane,” “(11-22) plane,” “(20-21) plane,” “(10-13) plane,” “(20-2-1)plane” or “(10-1-3) plane,” etc., of the present disclosure includes aplane that is tilted within ±5° in either direction with respect to itscorresponding non-tilted a-plane, +r-plane, −r-plane, S-plane, n-plane,R-plane, (11-22) plane, (20-21) plane, (10-13) plane, (20-2-1) plane, or(10-1-3) plane, etc.

Next, as shown in FIG. 4, a surface of the n-type gallium nitride-basedsemiconductor 41 is directly covered with a metal layer 42 having athickness of not less than 200 nm and not more than 1200 nm andcontaining silver (Ag) or aluminum (Al) or both of silver (Ag) andaluminum (Al). That is, in the present embodiment, the metal layer 42may be formed of Ag alone, may be formed of Al alone, or may be formedsuch that Ag and Al form an alloy at a given ratio. In the alloy as usedherein, Ag atoms and Al atoms do not need to be uniformly mixed, andeither Ag or Al atoms may be unevenly distributed. The definition of thealloy is the same in the second and third embodiments, as well. Further,the metal layer 42 is not limited to the alloy of Ag and Al. Forexample, the metal layer 42 may have a stacking structure of an Agsingle layer and an Al single layer. In the case of applying thestacking structure, the metal which directly contacts with the n-typegallium nitride-based semiconductor 41 may be any one of the Ag singlelayer and the Al single layer. Another metal layer, in a single layer ora plurality of layers, may be further provided on the metal layer 42.

Since directly covering the modified surface of the semiconductor 41,the metal layer 42 is firmly connected to the semiconductor 41 at aninterface 43. Thus, the metal layer 42 is strong against disturbances,such as heat and stress. The maximum length of each crystal graincontained in the metal layer 42 at the interface 43 in the in-planedirection is not less than 200 nm and not more than 1200 nm. Further,the thickness of the crystal grain contained in the metal layer 42 isalso not less than 200 nm and not more than 1200 nm. The oxygenconcentration in the metal layer 42 is low. This may be because sincethe crystal grain is relatively large in size, there are less grainboundaries which allows gas to pass through the metal layer 42. On theother hand, the oxygen concentration at the interface 43 is high due tothe surface modification. The oxygen concentration at the interface 43is not less than 30 times and not more than 200 times the oxygenconcentration in the metal layer 42. For example, the oxygenconcentration at the interface 43 is not less than 3×10²⁰ cm⁻³ and notmore than 2×10²¹ cm⁻³, and the oxygen concentration in the metal layer42 is not more than 1×10¹⁹ cm⁻³. As described above, the metal layer 42is firmly connected to the semiconductor 41 at the interface 43. Thus,the arithmetic average roughness Ra of the interface 43 of the metallayer 42 in the in-plane direction is not less than 0.27 nm and not morethan 2.65 nm in the case of a reference length of 3.5 μm.

In the first embodiment, the n-type gallium nitride-based semiconductor41 may have an n-type conductivity at least at only a surface partcontacting with the metal layer 42.

Second Embodiment

A gallium nitride-based semiconductor light-emitting device of thesecond embodiment will be described below with reference to FIG. 5A.

FIG. 5A shows a cross-sectional structure of a gallium nitride-basedsemiconductor light-emitting device formed on a gallium nitride-basedsemiconductor having a nonpolar plane or a semi-polar plane as a growthsurface according to the second embodiment.

As shown in FIG. 5A, a nitride semiconductor light-emitting device 55 ofthe present embodiment includes: a substrate 51 formed of GaN and havingan upper surface and a lower surface which are crystal planes other thanthe c-plane; an n-type gallium nitride-based semiconductor 41, a galliumnitride-based semiconductor active layer 52, and a p-type galliumnitride-based semiconductor 53 sequentially formed on the substrate 51;a p-side electrode 54 formed on and in contact with the p-type galliumnitride-based semiconductor 53; and a metal layer 42 formed on and incontact with an exposed portion of the n-type gallium nitride-basedsemiconductor 41. In this embodiment, as well, the n-type galliumnitride-based semiconductor 41 and the metal layer 42 have an interface43, as in the first embodiment.

In the present embodiment, the substrate 51 on which the n-type galliumnitride-based semiconductor 41 is epitaxially grown may be an m-planeGaN substrate, or may be an m-plane GaN layer on a dissimilar substrate,such as an m-plane GaN layer on an m-plane silicon carbide (SiC)substrate and an m-plane GaN layer on an r-plane sapphire substrate.Further, the surface of the substrate 51 is not limited to the m-plane,and may be a crystal plane which produces significant surfacemodification effects on the n-type gallium nitride-based semiconductor41. For example, a plane of which a principal plane is tilted at theangle of not less than 18° and not more than 90° with respect to thec-plane may be used as the substrate 51. For example, the principalplane may be m-plane, a-plane, +r-plane, −r-plane, S-plane, n-plane,R-plane, (11-22) plane, (20-21) plane, (10-13) plane, (20-2-1) plane, or(10-1-3) plane.

The n-type gallium nitride-based semiconductor 41 is formed of n-typeAl_(u)Ga_(v)In_(w)N (where u+v+w=1, u≧0, v>0, w≧0), for example. Forexample, silicon (Si) can be used as an n-type dopant.

The p-type gallium nitride-based semiconductor 53 is formed of a p-typeAl_(s)Ga_(t)N semiconductor (where s+t=1, s≧0, t>0), for example. Forexample, magnesium (Mg) can be used as a p-type dopant. Instead of usingMg as the p-type dopant, zinc (Zn) or beryllium (Be) can be used, forexample. In the p-type gallium nitride-based semiconductor 53, Alcomposition s may be uniform in the thickness direction, or thecomposition s may be varied in a continuous manner or a stepwise manner.The thickness of the p-type gallium nitride-based semiconductor 53 is,for example, about not less than 0.05 μm and not more than 2 μm. Aportion close to the upper surface of the p-type gallium nitride-basedsemiconductor 53, that is, a portion close to the interface with thep-side electrode 54, can be formed of semiconductor in which Alcomposition s is zero, that is, GaN. Further, in this case, a p-typeimpurity may be contained in the GaN in a high concentration, and thehigh-concentration region can serve as a contact layer.

The gallium nitride-based semiconductor active layer 52 has a multiplequantum well (MQW) structure of GaInN/GaInN, in which, for example, awell layer formed of Ga_(1-x)In_(x)N with a thickness of about not lessthan 3 nm and not more than 20 nm, and a barrier layer formed ofGa_(1-y)In_(y)N with a thickness of about not less than 5 nm and notmore than 30 nm (where 0≦y<x<1) are alternately layered.

The wavelength of light emitted from the nitride semiconductorlight-emitting device 55 is determined by an In composition x of theGa_(1-x)In_(x)N semiconductor that is a semiconductor composition of thewell layer. For example, a piezoelectric field is not generated in thegallium nitride-based semiconductor active layer 52 formed on them-plane. Thus, even if the In composition x is large, a decrease inluminous efficiency can be reduced.

The metal layer 42 is formed of a layer which contains at least one ofAg or Al, for example. That is, in the present embodiment, the metallayer 42 which serves as an n-side electrode may be formed of Ag alone,may be formed of Al alone, or may be formed such that Ag and Al form analloy at a given ratio. Further, the metal layer 42 may have a stackingstructure of an Ag single layer and an Al single layer. In the case ofapplying the stacking structure, the metal which directly contacts withthe n-type gallium nitride-based semiconductor 41 may be any one of theAg single layer and the Al single layer.

In one embodiment, the p-side electrode 54 covers almost entire surfaceof the p-type gallium nitride-based semiconductor 53. The p-sideelectrode 54 has a stacking structure (Pd/Pt) including a palladium (Pd)layer and a platinum (Pt) layer, or a stacking structure (Mg/Ag)including a magnesium (Mg) layer and a silver (Ag) layer.

(First Variation of Second Embodiment)

As the first variation, an undoped GaN layer 56 may be formed betweenthe gallium nitride-based semiconductor active layer 52 and the p-typegallium nitride-based semiconductor 53, as in the nitride semiconductorlight-emitting device 55 shown in FIG. 5B.

(Second Variation of Second Embodiment)

Further, as the second variation, a p-AlGaN layer 57 may be formed inthe p-type gallium nitride-based semiconductor 53, as in the nitridesemiconductor light-emitting device 55 shown in FIG. 5C. The p-AlGaNlayer 57 provided in the p-type gallium nitride-based semiconductor 53can reduce overflow of electrons injected during the operation of thelight-emitting element.

(Manufacturing Method)

Next, a method for manufacturing the nitride semiconductorlight-emitting device 55 of the present embodiment will be describedwith reference to FIG. 5A.

First, an n-type gallium nitride-based semiconductor 41 is epitaxiallygrown on a growth surface of the substrate 51 formed of n-type GaN, thegrowth surface being a crystal plane other than the c-plane, by metalorganic chemical vapor deposition (MOCVD) method. For example, Si isused as an n-type dopant, and TMG(Ga(CH₃)₃) as a group-III source andNH₃ as a group-V source are supplied as materials, and a growthtemperature is set to about not less than 900° C. and not more than1100° C. As a result, the n-type gallium nitride-based semiconductor 41formed of GaN and having a thickness of about not less than 1 μm and notmore than 3 μm is formed on the substrate 51.

Next, the gallium nitride-based semiconductor active layer 52 is formedon the n-type gallium nitride-based semiconductor 41. The galliumnitride-based semiconductor active layer 52 has, for example, a multiplequantum well (MQW) structure of GaInN/GaN, in which a Ga_(1-x)In_(x)Nwell layer with a thickness of 15 nm and a GaN barrier layer with athickness of 30 nm are alternately layered. A growth temperature whenforming the Ga_(1-x)In_(x)N well layer can be reduced to 800° C. toimprove efficiencies in incorporating In. The light emission wavelengthis selected according to purpose of the nitride semiconductorlight-emitting device 55, and the In composition x according to thewavelength can be determined. For example, to obtain the wavelength of450 nm (blue), the In composition x is determined to be not less than0.18 and not more than 0.2. To obtain the wavelength of 520 nm (green),the In composition x is determined to be not less than 0.29 and not morethan 0.31. To obtain the wavelength of 630 nm (red), the In compositionx is determined to be not less than 0.43 and not more than 0.44.

In the case of forming the undoped GaN layer 56 having a thickness, forexample, of not less than 15 nm and not more than 50 nm on the galliumnitride-based semiconductor active layer 52, as in the first variationshown in FIG. 5B, the p-type gallium nitride-based semiconductor layer53 is formed on the undoped GaN layer 56. In forming the p-type galliumnitride-based semiconductor layer 53, Cp₂Mg (cyclopentadienylmagnesium), for example, is used as a p-type dopant, and TMG and NH₃ aresupplied as materials. As a result, the p-type gallium nitride-basedsemiconductor 53 formed of p-type GaN having a thickness of about notless than 50 nm and not more than 300 nm can be formed at a growthtemperature of about not less than 900° C. and not more than 1100° C.

Further, the p-AlGaN layer 57 with a thickness of about not less than 15nm and not more than 30 nm in the p-type gallium nitride-basedsemiconductor layer 53, as in the second variation shown in FIG. 5C, canreduce overflow of electrons during operation.

Next, as shown in FIG. 5A, after the formation of the p-type galliumnitride-based semiconductor 53, a stacking semiconductor structureincluding the n-type gallium nitride-based semiconductor 41 epitaxiallygrown on the substrate, the gallium nitride-based semiconductor activelayer 52, and the p-type gallium nitride-based semiconductor 53, issubjected to heat treatment for about 20 minutes at a temperature ofabout not less than 800° C. and not more than 900° C.

Next, the stacking semiconductor structure is dry-etched using achlorine gas to remove part of each of the p-type gallium nitride-basedsemiconductor 53, the gallium nitride-based semiconductor active layer52, and the n-type gallium nitride-based semiconductor 41 and form arecess, thereby exposing part of the n-type gallium nitride-basedsemiconductor 41. As a result, a plane other than the c-plane, e.g., aplane tilted at the angle of not less than 18° and not more than 90°with respect to the c-plane, appears on the surface. This plane ism-plane, a-plane, +r-plane, −r-plane, S-plane, n-plane, R-plane, (11-22)plane, (20-21) plane, (10-13) plane, (20-2-1) plane, or (10-1-3) plane.

Next, the exposed surface of the n-type gallium nitride-basedsemiconductor 41 is exposed to an oxygen plasma atmosphere, for example,to modify the exposed portion of the surface of the n-type galliumnitride-based semiconductor 41. As a result, the hydrophilicity(wettability) of the exposed portion of the surface of the n-typegallium nitride-based semiconductor 41 is increased. After that, themetal layer 42 is formed on and in contact with the exposed portion ofthe n-type gallium nitride-based semiconductor 41. As the metal layer42, a layer containing Ag or Al or both of Ag and Al is formed, forexample.

Subsequently, the p-side electrode 54 is formed on and in contact withthe surface of the p-type gallium nitride-based semiconductor 53. Forexample, a Pd/Pt layer or an Mg/Ag layer can be used as the p-sideelectrode 54. After that, heat treatment is performed to cause alloyingof the metal layer 42 and the n-type gallium nitride-based semiconductor41, and cause alloying of the p-side electrode 54 and the p-type galliumnitride-based semiconductor 53. The order of formation of the metallayer 42 to be an n-side electrode and the p-side electrode 54 is notspecifically decided.

After that, the lower surface of the substrate 51 is ground to athickness of about 50 μm to 300 μm to reduce the thickness of thesubstrate 51. The thickness reduction of the substrate 51 not only leadsto easy dicing of the substrate 51, but also achieves a reduction inlight that is absorbed in the nitride semiconductor light-emittingdevice 55.

The thus obtained nitride semiconductor light-emitting device 55 isdivided into chips by dicing, and each of the chips is mounted on amount substrate that is a substrate formed of alumina (aluminum oxide),aluminum nitride (AlN), or resin. Further, in the case where silicon(Si) or germanium (Ge), is used as the mount substrate, a mount surfaceof the mount substrate can be covered with an insulating film. Aninterconnect can be arranged according to the shape of the electrode ofthe nitride semiconductor light-emitting device 55. Cu, Au, Ag, or Alcan be used as the interconnect. These materials can be applied onto themount substrate by sputtering method or plating method.

Third Embodiment

A gallium nitride-based semiconductor light-emitting device of the thirdembodiment will be described below with reference to FIG. 6A.

FIG. 6A shows a cross-sectional structure of a gallium nitride-basedsemiconductor light-emitting device 55A of the third embodiment, whichis formed on a gallium nitride-based semiconductor having a nonpolarplane or a semi-polar plane as a growth surface.

In the present embodiment, a substrate 51 is formed of n-type GaN havinga surface in a plane orientation in which effective surface modificationcan be made. For example, an upper surface and a lower surface which aregrowth surfaces of the substrate 51 are crystal planes other than thec-plane, and may be crystal planes tilted at the angle of not less than18° and not more than 90° with respect to the c-axis, and may bem-plane, a-plane, +r-plane, −r-plane, S-plane, n-plane, R-plane, (11-22)plane, (20-21) plane, (10-13) plane, (20-2-1) plane, or (10-1-3) plane.

In the third embodiment, a metal layer 42 is provided on a surface ofthe substrate 51 to be opposite to the n-type gallium nitride-basedsemiconductor 41. The metal layer 42 and the substrate 51 have aninterface 43. Further, the metal layer 42 which is an n-side electrodeserves as a reflector. A transparent electrode is used as the p-sideelectrode 54.

(First Variation of Third Embodiment)

As the first variation, an undoped GaN layer 56 may be formed betweenthe gallium nitride-based semiconductor active layer 52 and the p-typegallium nitride-based semiconductor 53, as in the nitride semiconductorlight-emitting device 55A shown in FIG. 6B.

(Second Variation of Third Embodiment)

Further, as the second variation, the p-AlGaN layer 57 may be formed inthe p-type gallium nitride-based semiconductor 53, as in the nitridesemiconductor light-emitting device 55A shown in FIG. 6C. The p-AlGaNlayer 57 provided in the p-type gallium nitride-based semiconductor 53can reduce overflow of electrons injected during the operation of thelight-emitting element.

(Manufacturing Method)

In the third embodiment, the nitride-based semiconductor light-emittingdevice 55A having a crystal plane, other than the c-plane, as a growthsurface is manufactured.

Similar to the second embodiment, an n-type gallium nitride-basedsemiconductor 41, a gallium nitride-based semiconductor active layer 52,and a p-type gallium nitride-based semiconductor 53 are epitaxiallygrown by MOCVD method, sequentially on a growth surface of the substrate51 having a GaN layer which has a crystal plane, other than the c-plane,as a growth surface. After that, heat treatment is performed at atemperature about not less than 800° C. and not more than 900° C. forabout 20 minutes.

In the first variation, as shown in FIG. 6B, an undoped GaN layer 56equivalent to the undoped GaN layer 56 of the second embodiment isformed on the gallium nitride-based semiconductor active layer 52. Thus,the p-type gallium nitride-based semiconductor 53 is formed on theundoped GaN layer 56.

In the second variation, as shown in FIG. 6C, the p-AlGaN layer 57equivalent to the p-AlGaN layer 57 of the second embodiment is formed inthe p-type gallium nitride-based semiconductor 53.

Next, in a manner similar to the second embodiment, the lower surface ofthe substrate 51 is ground to reduce the thickness. A ground surface ofthe substrate 51 of which the thickness has been reduced may be a planetilted within ±5° with respect to any one of m-plane, a-plane, +r-plane,−r-plane, S-plane, n-plane, R-plane, (11-22) plane, (20-21) plane,(10-13) plane, (20-2-1) plane, and (10-1-3) plane. Subsequently, theground surface is exposed to an oxygen plasma atmosphere, for example,thereby modifying the ground surface of the substrate 51 and increasingthe hydrophilicity of the ground surface. After that, a metal layer 42is formed to be in contact with part of the ground surface of thesubstrate 51. As the metal layer 42, a layer containing Ag or Al or bothof Ag and Al is formed, for example. In a manner similar to the secondembodiment, the metal layer 42 may be an alloy of Ag and Al, or may havea stacking structure of an Ag single layer and an Al single layer,either of which is brought into contact with the ground surface of thesubstrate 51.

Subsequently, the p-side electrode 54 is formed on and in contact withthe surface of the p-type gallium nitride-based semiconductor 53. As thep-side electrode 54, a Pd/Pt layer or an Mg/Ag layer having a thicknesswhich allows transmission of light, or an indium tin oxide (ITO)electrode, etc., which transmits light, is formed. After that, heattreatment is performed to cause alloying of a connecting portion betweenthe metal layer 42 and the substrate 51, and a connecting portionbetween the p-side electrode 54 and the p-type gallium nitride-basedsemiconductor 53.

The thus obtained nitride semiconductor light-emitting device 55A isdivided into chips by dicing, and each of the chips is mounted on amount substrate thereafter.

EXAMPLES First Example

As the first example, two m-plane GaN substrates were prepared, andupper and lower surfaces of each of the two substrates were polished tomirror surfaces. Subsequently, one of the two substrates was subjectedto organic cleaning, and was thereafter cleaned with buffered HF (BHF).The other substrate was exposed to an oxygen plasma atmosphere to modifyits surface. Oxygen plasma was generated using a high density plasmaetching apparatus (NE-500) of an inductively-coupled discharge type thatwas produced by ULVAC, Inc., under process condition of antenna power of500 W and bias power of 30 W. Further, an oxygen flow rate was set to 20ml/min (0° C., 1 atmospheric pressure), and a pressure was set to 0.6Pa, with the process time of 30 seconds. After that, an aluminum (Al)layer having a thickness of 500 nm was formed on a principal plane ofeach of the two substrates by electron beam evaporation method. As aresult, the structure shown in FIG. 4 was manufactured.

Next, a resist solution was applied onto the Al layer, and atransmission line model (TLM) pattern was formed in the resist filmusing an exposure apparatus and developer. Subsequently, the TLM patternwas transferred to the Al layer using Al etchant, and the resist filmwas removed by organic cleaning. As a result, TLM patterns formed in twotypes of Al layers were manufactured.

Each of the Al layers thus manufactured was observed with an opticalmicroscope before heat treatment, and the results are shown in FIG. 7A,FIG. 8A, FIG. 9A, and FIG. 10A. FIG. 7A is a comparative example,showing an optical microscope image of a surface of an Al layer beforeheat treatment which was formed on an unmodified surface of an m-planen-type GaN. FIG. 8A is a comparative example, showing an opticalmicroscope image of an interface, before heat treatment, of an Al layerwith an unmodified surface of an m-plane n-type GaN on which the Allayer was formed, and the image was observed from an m-plane n-type GaNside. FIG. 9A is the inventive example, showing an optical microscopeimage of a surface of the Al layer before heat treatment which wasformed on a modified surface of the m-plane n-type GaN. FIG. 10A is theinventive example, showing an optical microscope image of a surface ofan interface, before heat treatment, of the Al layer with a modifiedsurface of the m-plane n-type GaN on which the Al layer was formed, andthe image was observed from an m-plane n-type GaN side.

Although there are a lot of uneven portions in the Al surface of FIG.7A, all of the images show that relatively planar Al surface and Al—GaNinterface were manufactured at a point before heat treatment.

On the other hand, optical microscope images of the above two sampleswhich were subjected to heat treatment in an nitrogen (N₂) atmospherefor 10 minutes at a temperature of 500° C. are shown in FIG. 7B, FIG.8B, FIG. 9B, and FIG. 10B. FIG. 7B is a comparative example, showing anoptical microscope image of a surface of an Al layer after heattreatment which was formed on an unmodified surface of an m-plane n-typeGaN. FIG. 8B is a comparative example, showing an optical microscopeimage of an interface, after heat treatment, of an Al layer with anunmodified surface of an m-plane n-type GaN on which the Al layer wasformed, and the image was observed from an m-plane n-type GaN side. FIG.9B is the inventive example, showing an optical microscope image of asurface of the Al layer after heat treatment which was formed on amodified surface of the m-plane n-type GaN. FIG. 10B is the inventiveexample, showing an optical microscope image of an interface, after heattreatment, of the Al layer with a modified surface of the m-plane n-typeGaN on which the Al layer was formed, and the image was observed from anm-plane n-type GaN side. According to these images, the samples withunmodified surfaces have a lot of uneven portions not only in the Al—GaNinterface, but also in the Al surface, as shown in FIGS. 7A-7B and FIGS.8A-8B, whereas in the samples with modified surfaces, the Al—GaNinterface and the Al surface are as planar as those of the samplesbefore heat treatment, as shown in FIGS. 9A-9B and FIGS. 10A-10B. It isknown from this that the Al layer formed on the modified surface of then-type GaN is firmly connected to the GaN layer, and is therefore strongagainst disturbances, such as heat.

Next, results of evaluation of current-voltage (1-V) characteristics ofthe above two samples are shown in FIG. 11 and FIG. 12. FIG. 11 relatesto a comparative example, showing a result of I-V measurement of an Allayer which was formed on an unmodified surface of an m-plane n-type GaNand was given a pattern. FIG. 12 relates to the inventive example,showing a result of I-V measurement of the Al layer which was formed ona modified surface of the m-plane n-type GaN and was given a pattern.The results show that the Al layer on the modified surface of the n-typeGaN shown in FIG. 12 exhibits similar electrical characteristics as theAl layer on the unmodified surface of the n-type GaN shown in FIG. 11.It is known from this that the Al layer on the modified surface of then-type GaN also exhibits a low contact resistance with respect to theGaN layer.

Second Example

As the second example, two m-plane n-type GaN substrates were prepared,and upper and lower surfaces of each of the two substrates were polishedto mirror surfaces. Subsequently, one of the two substrates wassubjected to organic cleaning, and was thereafter cleaned with bufferedHF (BHF). The other substrate was exposed to an oxygen plasma atmosphereto modify its surface. Oxygen plasma was generated using a high densityplasma etching device (NE-500) of an inductively-coupled discharge typethat was produced by ULVAC, Inc., under process condition of antennapower of 500 W and bias power of 30 W. Further, an oxygen flow rate wasset to 20 ml/min (0° C., 1 atmospheric pressure), and a pressure was setto 0.6 Pa, with the process time of 30 seconds. After that, a silver(Ag) layer having a thickness of 500 nm was formed on a principal planeof each of the two substrates by electron beam evaporation method. As aresult, the structure shown in FIG. 4 was manufactured.

Next, a resist solution was applied onto the Ag layer, and a TLM patternwas formed in the resist film using an exposure device and developer.Subsequently, the TLM pattern was transferred to the Ag layer using Agetchant, and the resist film was removed by organic cleaning. As aresult, TLM patterns formed in two types of Al layers were manufactured.

Results of evaluation of current-voltage (1-V) characteristics of theabove two samples are shown in FIG. 13 and FIG. 14. FIG. 13 relates to acomparative example, showing a result of I-V measurement of an Ag layerwhich was formed on an unmodified surface of an m-plane n-type GaN andwas given a pattern. FIG. 14 relates to the inventive example, showing aresult of I-V measurement of the Ag layer which was formed on a modifiedsurface of the m-plane n-type GaN and was given a pattern. The resultsshow that an ohmic contact cannot be formed with the Ag layer on theunmodified surface of the n-type GaN shown in FIG. 13, whereas the Aglayer on the modified surface of the n-type GaN exhibits good electricalcharacteristics, that is, an ohmic contact was formed. This means thatthe Ag layer which, in general, cannot be used as an electrode of agallium nitride-based semiconductor, can be used on the n-type GaN bymodifying the surface of the n-type GaN.

Next, results of measurement of reflection coefficients of the above twosamples after heat treatment in a nitrogen atmosphere for 20 minutes ata temperature of 500° C., are shown in FIG. 15 and FIG. 16. FIG. 15relates to a comparative example, showing a result of measurement of areflection coefficient of an interface, after heat treatment, of an Aglayer with an unmodified surface of an m-plane n-type GaN on which theAg layer was formed, and the measurement was performed from the m-planen-type GaN side. FIG. 16 relates to the inventive example, showing aresult of measurement of a reflection coefficient of an interface, afterheat treatment, of the Ag layer with a modified surface of the m-planen-type GaN on which the Ag layer was formed, and the measurement wasperformed from the m-plane n-type GaN side. As a measurement device, anultraviolet visible spectrophotometer (V-570) manufactured by JASCOCorporation into which an absolute reflection coefficient measurementapparatus (ARN-475) was incorporated was used. In the measurement, a GaNsubstrate with its both surfaces polished, was irradiated with lightwith a wavelength of from 350 nm to 800 nm from a mirror surface side onwhich an Ag layer was not formed, and the light reflected by theinterface between the GaN substrate and the Ag layer was received on themirror surface side on which the Ag layer was not formed. In thecomparative example where the surface was not modified, surface plasmonresonance (SPR) absorption due to generation of Ag nanoparticles wasobserved in a wavelength range of from 450 nm to 500 nm as shown in FIG.15. However, in the inventive example where the surface was modified,almost no SPR absorption was observed as shown in FIG. 16, from which itis known that the Ag layer on the modified surface of the n-type GaN isfirmly connected to the GaN layer and is strong against disturbances,such as heat, and therefore that generation of nanoparticles is notpromoted.

Next, optical microscope images of the above two samples before andafter heat treatment in a nitrogen atmosphere for 20 minutes at atemperature of 500° C. are shown in FIG. 17A, FIG. 17B, FIG. 18A, andFIG. 18B.

FIG. 17A is a comparative example, showing an optical microscope imageof an interface, before heat treatment, of an Ag layer with anunmodified surface of an m-plane n-type GaN on which the Ag layer wasformed, and the image was observed from the m-plane n-type GaN side.FIG. 17B is a comparative example, showing an optical microscope imageof an interface, after heat treatment, of an Ag layer with an unmodifiedsurface of an m-plane n-type GaN on which the Ag layer was formed, andthe image was observed from the m-plane n-type GaN side. FIG. 18A is theinventive example, showing an optical microscope image of an interface,before heat treatment, of the Ag layer with a modified surface of them-plane n-type GaN on which the Ag layer was formed, and the image wasobserved from the m-plane n-type GaN side. FIG. 18B is the inventiveexample, showing an optical microscope image of an interface, after heattreatment, of the Ag layer with a modified surface of the m-plane n-typeGaN on which the Ag layer was formed, and the image was observed fromthe m-plane n-type GaN side. According to these images, the sample withthe unmodified surface has uneven portions in the Ag—GaN interface, asshown in FIG. 17B, whereas in the sample with modified surface, theAg—GaN interface is as planar as that of the sample before heattreatment, as shown in FIG. 18B. It is known from this that the Ag layeron the modified surface of the n-type GaN is firmly connected to the GaNlayer, and is therefore strong against disturbances, such as heat.

Next, results of observation of cross-sectional TEM images of the abovetwo samples on which heat treatment was performed are shown in FIG. 19,FIG. 20, FIG. 21, and FIG. 22. FIG. 19 is a comparative example, showinga cross-sectional TEM image of an interface, after heat treatment, of anAg layer with an unmodified surface of an m-plane n-type GaN on whichthe Ag layer was formed. FIG. 20 is the inventive example, showing across-sectional TEM image of an interface, after heat treatment, of theAg layer with a modified surface of the m-plane n-type GaN on which theAg layer was formed. FIG. 21 is a comparative example, showing across-sectional TEM image of an interface, after heat treatment, of anAg layer with an unmodified surface of an m-plane n-type GaN on whichthe Ag layer was formed. FIG. 22 is the inventive example, showing across-sectional TEM image of an interface, after heat treatment, of theAg layer with a modified surface of the m-plane n-type GaN on which theAg layer was formed. FIG. 23 schematically illustrates thecross-sectional TEM image of FIG. 19. FIG. 24 schematically illustratesthe cross-sectional TEM image of FIG. 20. In the comparative examplewhere the surface was not modified, a lot of nanoparticles are presentin the Ag layer, and a maximum grain size is less than about 200 nm, asknown from FIG. 19 and FIG. 23. On the other hand, in the inventiveexample where the surface was modified, only four crystal grains arepresent in the Ag layer shown in FIG. 20 and FIG. 24, and a maximumlength of the crystal grain at the interface with the m-plane n-type GaNin an in-plane direction is not less than 200 nm and not more than 1200nm, as known from the drawings. Further, the thickness of each of thecrystal grains is not less than 200 nm, and each crystal grain extendsfrom the lower surface to the upper surface of the Ag layer. Further,the length of the largest crystal grain at the interface in the in-planedirection is not less than 600 nm, and the thickness thereof is not lessthan 400 nm. The thickness of the Ag layer, that is, the thickness ofthe crystal grains may be not more than about 1200 nm, depending on thethickness of the film.

Further, it is known from FIG. 21 showing a comparative example wherethe surface was not modified, that arithmetic average roughness Ra ofthe GaN—Ag interface is larger than 2.65 nm in the case of a referencelength of 3.5 μm. It is known from FIG. 22 showing the inventive examplewhere the surface was modified, that the arithmetic average roughness Raof the GaN—Ag interface is 0.27 nm in the case of a reference length of3.5 μm. These observations reveal that the Ag layer on the modifiedsurface of the n-type GaN is firmly connected to the GaN layer, and isstrong against disturbances, such as heat.

Next, secondary ion mass spectrometry (SIMS) was performed on the abovetwo samples on which heat treatment was performed, to find out that anoxygen concentration in the Ag layer formed on the unmodified surface ofthe m-plane n-type GaN was 4×10¹⁹ cm⁻³ and that an oxygen concentrationat the Ag—GaN interface was 5×10²⁰ cm⁻³. On the other hand, an oxygenconcentration in the Ag layer formed on the modified surface of them-plane n-type GaN was 1×10¹⁹ cm⁻³, and an oxygen concentration at theAg—GaN interface was not less than 3×10²⁰ cm⁻³ and not more than 2×10²¹cm⁻³. These observations revealed that the oxygen concentration at theinterface between the modified surface of the m-plane n-type GaN and theAg layer was not less than 30 times and not more than 200 times theoxygen concentration in the Ag layer.

Here, both Ag and Al have the same face-centered cubic lattice. Thus,the crystal grain size, the arithmetic average roughness, and the oxygenconcentration in the metal layer with respect to silver (Ag) in thesecond example described above also hold true for aluminum (Al) in thefirst example.

Other Embodiments

FIG. 25 is a flow chart showing an example method for manufacturing astructure including a gallium nitride-based semiconductor layer(Al_(x)Ga_(y)In_(z)N (where 0≦x<1, 0<y≦1, 0≦z<1, x+y+z=1)) and a metallayer according to another embodiment.

First, in step S0, a gallium nitride-based semiconductor layer with asurface that is a nonpolar plane or a semi-polar plane is prepared, asshown in FIG. 25. The gallium nitride-based semiconductor layer is ann-type GaN layer, for example.

Next, in step S1, the surface of the gallium nitride-based semiconductorlayer which is n-type and is a nonpolar plane or a semi-polar plane isexposed to oxygen to modify the surface.

Next, in step S2, a metal layer containing Ag or Al or both of Ag and Alis formed on the modified surface. Thus, the interface between thegallium nitride-based semiconductor layer and the metal layer containsoxygen. Further, the metal layer has a structure which contains acrystal grain extending from the lower surface to the upper surface ofthe metal layer.

Predetermined heat treatment may be performed after step S2. Forexample, heat treatment in a nitrogen atmosphere at a temperature of500° C. for about 10 to 20 minutes may be performed. Through this heattreatment, as well, the metal layer can have a structure which containsa crystal grain extending from the lower surface to the upper surface ofthe metal layer.

In the exposure processing of step S1, the surface of the galliumnitride-based semiconductor layer which is n-type and is a nonpolarplane or a semi-polar plane may be exposed to an atmosphere containingactive oxygen. Further, in the exposure processing of step S1, thesurface of the gallium nitride-based semiconductor layer which is n-typeand is a nonpolar plane or a semi-polar plane may be irradiated withultraviolet light, with the surface exposed to a gas or a liquidcontaining oxygen atoms or oxygen molecules. Further, in exposureprocessing of step S1, the surface of the gallium nitride-basedsemiconductor layer which is n-type and is a nonpolar plane or asemi-polar plane may be exposed to an atmosphere containing oxygenplasma.

FIG. 26 is a flow chart showing an example method for manufacturing agallium nitride-based semiconductor light-emitting device according toanother embodiment.

First, in step S10, a stacking semiconductor structure including alight-emitting layer and a p-type gallium nitride-based semiconductorlayer is formed on a gallium nitride-based semiconductor (e.g.,Al_(x)Ga_(y)In_(z)N (0≦x<1, 0<y≦1, 0≦z<1, x+y+z=1)) having a nonpolarplane or a semi-polar plane as a growth surface, as shown in FIG. 26.For example, the gallium nitride-based semiconductor is a substrate andan n-type gallium nitride-based semiconductor layer formed on thesubstrate. The substrate can be reduced in thickness or removed. Then-type gallium nitride-based semiconductor layer is, for example, ann-type GaN layer.

Next, in step S11, part of each of the stacking semiconductor structureand the gallium nitride-based semiconductor is removed to expose asurface of the gallium nitride-based semiconductor layer which is n-typeand is a nonpolar plane or a semi-polar plane.

Next, in step S12, the exposed surface is exposed to oxygen to modifythe surface.

Next, in step S13, a first metal layer is formed in contact with themodified surface. As the first metal layer, a metal containing Ag or Alor both of Ag and Al can be used. Further, a second metal layer isformed on and in contact with the p-type gallium nitride-basedsemiconductor layer. Either one of the first metal layer and the secondmetal layer can be formed first. Thus, the interface between the n-typeGaN and the first metal layer contains oxygen. Further, the first metallayer has a structure which contains a crystal grain extending from thelower surface to the upper surface of the first metal layer.

Predetermined heat treatment may be performed after step S13. Forexample, heat treatment in a nitrogen atmosphere at a temperature of500° C. for about 10 to 20 minutes may be performed. Through this heattreatment, as well, the first metal layer can have a structure whichcontains a crystal grain extending from the lower surface to the uppersurface of the first metal layer.

In the exposure processing of step S12, the surface of the galliumnitride-based semiconductor layer which is n-type and is a nonpolarplane or a semi-polar plane may be exposed to an atmosphere containingactive oxygen. Further, in the exposure processing of step S12, thesurface of the gallium nitride-based semiconductor layer which is n-typeand is a nonpolar plane or a semi-polar plane may be irradiated withultraviolet light, with the surface exposed to a gas or a liquidcontaining oxygen atoms or oxygen molecules. Further, in the exposureprocessing of step S12, the surface of the gallium nitride-basedsemiconductor layer which is n-type and is a nonpolar plane or asemi-polar plane may be exposed to an atmosphere containing oxygenplasma.

FIG. 27 is a flow chart showing an example method for manufacturing agallium nitride-based semiconductor light-emitting device according toanother embodiment.

First, in step S20, a stacking semiconductor structure including alight-emitting layer and a p-type gallium nitride-based semiconductorlayer is formed on a gallium nitride-based semiconductor (e.g.,Al_(x)Ga_(y)In_(z)N (0≦x<1, 0<y≦1, 0≦z<1, x+y+z=1)) having a nonpolarplane or a semi-polar plane as a growth surface, as shown in FIG. 27.For example, the gallium nitride-based semiconductor is a substrate andan n-type gallium nitride-based semiconductor layer formed on thesubstrate. The substrate can be reduced in thickness or removed. Thesubstrate and the n-type gallium nitride-based semiconductor layer aren-type GaN layers, for example. A surface of the n-type galliumnitride-based semiconductor layer on the side opposite to the side wherethe stacking semiconductor structure is formed, is n-type and is anonpolar plane or a semi-polar plane.

Next, in step S21, the surface opposite to the side where the stackingsemiconductor structure has been formed in step S20, is exposed tooxygen to modify the surface.

Next, in step S22, a first metal layer is formed in contact with themodified surface. Further, a second metal layer is formed on and incontact with the p-type gallium nitride-based semiconductor layer.

Steps S21 and S22 are similar to steps S12 and S13 described above.

As described above, according to other embodiments, the affinity betweenthe gallium nitride-based semiconductor and the metal layer becomesstronger, and the gallium nitride-based semiconductor and the metallayer form a definite interface, by modifying the surface of the galliumnitride-based semiconductor. As a result, not only a reflectioncoefficient, but also the reliability of the metal layer increase.Moreover, it is possible to provide a gallium nitride-basedsemiconductor light-emitting device in which an ohmic contact can beformed even between a gallium nitride-based semiconductor and a metallayer, e.g., Ag, between which an ohmic contact could not be formed byconventional techniques.

CONCLUSION

A preferable embodiment of the present invention derived from the abovedisclosure will be described below.

A nitride semiconductor light-emitting device comprising:

an n-type nitride semiconductor layer (41);

a p-type nitride semiconductor layer (53);

an active layer (52) sandwiched between the n-type nitride semiconductorlayer (41) and the p-type nitride semiconductor layer (53);

an n-side electrode layer (42); and

a p-side electrode layer (54) formed on a surface of the p-type nitridesemiconductor layer (53), wherein

each of the n-type nitride semiconductor layer (41), the active layer(52), and the p-type nitride semiconductor layer (53) has a principalplane of a nonpolar plane or a semi-polar plane,

the n-side electrode layer (42) has a first surface (42 a) and a secondsurface (42 b),

the first surface (42 a) is in contact with at least part of a surfaceof the n-type nitride semiconductor layer (41),

the second surface (42 b) is a surface opposite to the first surface (42a),

the n-side electrode layer (42) is formed of silver,

the part of the surface of the n-type nitride semiconductor layer (41)in contact with the n-side electrode layer (42) contains oxygen,

the n-side electrode layer (42) has a thickness of not less than 200 nmand not more than 1200 nm, and

the n-side electrode layer (42) includes a silver crystal grainextending from the first surface (42 a) to the second surface (42 b).

Another preferable embodiment of the present invention derived from theabove disclosure will be described below.

A nitride semiconductor light-emitting device comprising:

a n-type nitride semiconductor layer (41);

a p-type nitride semiconductor layer (53);

an active layer (52) sandwiched between the n-type nitride semiconductorlayer (41) and the p-type nitride semiconductor layer (53);

an n-side electrode layer (42); and

a p-side electrode layer (54) formed on a surface of the p-type nitridesemiconductor layer (53), wherein

each of the n-type nitride semiconductor layer (41), the active layer(52), and the p-type nitride semiconductor layer (53) has a principalplane of a nonpolar plane or a semi-polar plane,

the n-side electrode layer (42) has a first surface (42 a) and a secondsurface (42 b),

the first surface (42 a) is in contact with at least part of a surfaceof the n-type nitride semiconductor layer (41),

the second surface (42 b) is a surface opposite to the first surface (42a),

the n-side electrode layer (42) is formed of silver,

the part of the surface of the n-type nitride semiconductor layer (41)in contact with the n-side electrode layer (42) contains oxygen,

the n-side electrode layer (42) has a thickness of not less than 200 nmand not more than 1200 nm, and

the n-side electrode layer (42) includes a plurality of silver crystalgrains each extending from the first surface (42 a) to the secondsurface (42 b).

A structure and a manufacturing method of the structure, and a galliumnitride-based semiconductor light-emitting device using the structureand a manufacturing method of the device are applicable to the fields ofdisplay, lighting, and optical information.

What is claimed is:
 1. A structure, comprising: a gallium nitride-basedsemiconductor layer having an m-plane as a principal plane; and a silverlayer provided on the principal plane, wherein the principal plane hasan n-type conductivity, an interface between the gallium nitride-basedsemiconductor layer and the silver layer contains oxygen, the silverlayer includes a crystal grain extending from a lower surface to anupper surface of the silver layer, and an oxygen concentration at theinterface between the gallium nitride-based semiconductor layer and thesilver layer is not less than 30 times and not more than 200 times anoxygen concentration in the silver layer.
 2. The structure of claim 1,wherein surface plasmon resonance absorption is not observed in lightreflected by the interface between the gallium nitride-basedsemiconductor layer and the silver layer, within a wavelength range offrom 450 nm to 500 nm.
 3. The structure of claim 1, wherein the silverlayer has a thickness of not less than 200 nm.
 4. The structure of claim1, wherein the silver layer has a thickness of not more than 1200 nm. 5.The structure of claim 1, wherein the crystal grain has a maximum lengthof not less than 200 nm.
 6. The structure of claim 1, wherein thecrystal grain has a maximum length of not more than 1200 nm.
 7. Thestructure of claim 1, wherein an arithmetic average roughness Ra of theinterface of the silver layer with the gallium nitride-basedsemiconductor layer in an in-plane direction is not less than 0.27 nmand not more than 2.65 nm in the case of a reference length of 3.5 μm.8. A gallium nitride-based semiconductor light-emitting device,comprising: the structure of claim 1; a p-type gallium nitride-basedsemiconductor layer, and a light-emitting layer sandwiched between thegallium nitride-based semiconductor layer and the p-type galliumnitride-based semiconductor layer, wherein the gallium nitride-basedsemiconductor layer is an n-type gallium nitride-based semiconductorlayer, and the silver layer is an electrode for the n-type galliumnitride-based semiconductor layer.
 9. A structure, comprising: a galliumnitride-based semiconductor layer having an m-plane as a principalplane; and a silver layer provided on the principal plane, wherein theprincipal plane has an n-type conductivity, an interface between thegallium nitride-based semiconductor layer and the silver layer containsoxygen, the silver layer includes a crystal grain extending from a lowersurface to an upper surface of the silver layer, an oxygen concentrationat the interface between the gallium nitride-based semiconductor layerand the silver layer is not less than 3×10²⁰ cm⁻³ and not more than2×10²¹ cm⁻³, and an oxygen concentration in the silver layer is not morethan 1×10¹⁹ cm⁻³.
 10. The structure of claim 9, wherein surface plasmonresonance absorption is not observed in light reflected by the interfacebetween the gallium nitride-based semiconductor layer and the silverlayer, within a wavelength range of from 450 nm to 500 nm.
 11. Thestructure of claim 9, wherein the silver layer has a thickness of notless than 200 nm.
 12. The structure of claim 9, wherein the silver layerhas a thickness of not more than 1200 nm.
 13. The structure of claim 9,wherein the crystal grain has a maximum length of not less than 200 nm.14. The structure of claim 9, wherein the crystal grain has a maximumlength of not more than 1200 nm.
 15. The structure of claim 9, whereinan arithmetic average roughness Ra of the interface of the silver layerwith the gallium nitride-based semiconductor layer in an in-planedirection is not less than 0.27 nm and not more than 2.65 nm in the caseof a reference length of 3.5 μm.
 16. A gallium nitride-basedsemiconductor light-emitting device, comprising: the structure of claim9; a p-type gallium nitride-based semiconductor layer, and alight-emitting layer sandwiched between the gallium nitride-basedsemiconductor layer and the p-type gallium nitride-based semiconductorlayer, wherein the gallium nitride-based semiconductor layer is ann-type gallium nitride-based semiconductor layer, and the silver layeris an electrode for the n-type gallium nitride-based semiconductorlayer.